Flip chip mounting is an increasingly popular technique for directly electrically connecting an integrated circuit chip to a substrate such as a circuit board. In this configuration, the active face of the chip is mounted face down, or “flipped” on the substrate. The electrical bond pads on the flip chip are aligned with corresponding electrical bond pads on the substrate, with the chip and substrate bond pads electrically connected by way of an electrically conductive material. The flip chip mounting technique eliminates the use of bond wires between a chip or chip package and the substrate, substantially increases the reliability of the chip-to-substrate bond.
As a means for mounting integrated circuit chips to a substrate, there has been known a number of methods which form solder portions, such as solder bumps and solder precoats, on the integrated circuit chip and joins the integrated circuit chip to a substrate by means of the solder portions. Typically, the soldering process involves applying a flux to substrate and mounting the integrated circuit chip to a substrate, and heating and melting the solder to join the solder portions. After the solder joints have been formed, the assembly is subjected to cleaning to remove flux residues to enhance the reliability after the mounting.
Additionally the resulting assembly typically undergoes further thermal cycling during additional assembly operations. The final assembly also is exposed to wide temperature changes in the service environment. The integrated circuit chip is typically silicon and the substrate may be epoxy, or ceramic. Both the material of the integrated circuit chip and the substrate frequently have thermal expansion coefficients that are different from one another, and are also different from the thermal expansion coefficient of solder. The differential expansion that the assembly invariably undergoes results in stresses on the solder bonds which can cause stress cracking and ultimately failure of the electrical path through the solder bond. To avoid solder bond failures due to mechanical stress, the gap between the surfaces joined by the bond is typically filled with an underfill material.
Conventionally, the underfill material is dispensed between the chip and the substrate. The underfill material is typically provided as a liquid adhesive resin that can be dried or polymerized. The underfill material provides enhanced mechanical adhesion and mechanical and thermal stability between the flip chip and the substrate, and inhibits environmental attack of chip and substrate surfaces. The underfill material also fills the gaps between the bumped electronic parts and the board to reinforce the joints. The underfill resin is then hardened by heat treatment, thus completing the mounting process.
The mounting process described above, however, poses the following problems as the use of such solvents as fluorocarbon are not considered environmentally safe. Further, the cleaning process after soldering has become complicated and risen in cost, which, combined with on-going reductions in the size of integrated circuit chip, has contributed to making the cleaning process technically difficult. As to the underfill resin, since the gaps between the integrated circuit chip and the substrate is minimized to a need for smaller components filling of the underfill after the mounting of electronic components difficult, resulting in unstable quality of the assembly. In addition to this quality problem, the above conventional mounting method has another problem that it requires two heating processes for the mounting of each component, one for soldering and one for hardening the resin, thus complicating the process. Additionally, in some cases entrapped air, or incomplete wetting of the surfaces of the space being filled, inhibits flow or prevents wicking, causing voids in the underfill. The above method also has another problem that it requires two heating processes. One for mounting the integrated circuit chip to the substrate and the other for hardening the resin, thereby complicating the process and the time for manufacturing the assembly.
Therefore, there is a need in the flip-chip bonding industry to have a process that substantially reduces cure time for the underfill and at the same time having a more reliable bond.